Identification of resynthesized /hVd/ utterances: effects of formant contour.

The purpose of this study was to examine the role of formant frequency movements in vowel recognition. Measurements of vowel duration, fundamental frequency, and formant contours were taken from a database of acoustic measurements of 1668 /hVd/ utterances spoken by 45 men, 48 women, and 46 children [Hillenbrand et al., J. Acoust. Soc. Am. 97, 3099-3111 (1995)]. A 300-utterance subset was selected from this database, representing equal numbers of 12 vowels and approximately equal numbers of tokens produced by men, women, and children. Listeners were asked to identify the original, naturally produced signals and two formant-synthesized versions. One set of "original formant" (OF) synthetic signals was generated using the measured formant contours, and a second set of "flat formant" (FF) signals was synthesized with formant frequencies fixed at the values measured at the steadiest portion of the vowel. Results included: (a) the OF synthetic signals were identified with substantially greater accuracy than the FF signals; and (b) the naturally produced signals were identified with greater accuracy than the OF synthetic signals. Pattern recognition results showed that a simple approach to vowel specification based on duration, steady-state F0, and formant frequency measurements at 20% and 80% of vowel duration accounts for much but by no means all of the variation in listeners' labeling of the three types of stimuli.     

The direct and indirect roles of fundamental frequency in vowel perception

Several experiments have found that changing the intrinsic f0 of a vowel can have an effect on perceived vowel quality. It has been suggested that these shifts may occur because f0 is involved in the specification of vowel quality in the same way as the formant frequencies. Another possibility is that f0 affects vowel quality indirectly, by changing a listener's assumptions about characteristics of a speaker who is likely to have uttered the vowel. In the experiment outlined here, participants were asked to listen to vowels differing in terms of f0 and their formant frequencies and report vowel quality and the apparent speaker's gender and size on a trial-by-trial basis. The results presented here suggest that f0 affects vowel quality mainly indirectly via its effects on the apparent-speaker characteristics; however, f0 may also have some residual direct effects on vowel quality. Furthermore, the formant frequencies were also found to have significant indirect effects on vowel quality by way of their strong influence on the apparent speaker.     

Effects of consonant environment on vowel formant patterns

A significant body of evidence has accumulated indicating that vowel identification is influenced by spectral change patterns. For example, a large-scale study of vowel formant patterns showed substantial improvements in category separability when a pattern classifier was trained on multiple samples of the formant pattern rather than a single sample at steady state [J. Hillenbrand et al., J. Acoust. Soc. Am. 97, 3099-3111 (1995)]. However, in the earlier study all utterances were recorded in a constant /hVd/ environment. The main purpose of the present study was to determine whether a close relationship between vowel identity and spectral change patterns is maintained when the consonant environment is allowed to vary. Recordings were made of six men and six women producing eight vowels (see text) in isolation and in CVC syllables. The CVC utterances consisted of all combinations of seven initial consonants (/h,b,d,g,p,t,k/) and six final consonants (/b,d,g,p,t,k/). Formant frequencies for F1-F3 were measured every 5 ms during the vowel using an interactive editing tool. Results showed highly significant effects of phonetic environment. As with an earlier study of this type, particularly large shifts in formant patterns were seen for rounded vowels in alveolar environments [K. Stevens and A. House, J. Speech Hear. Res. 6, 111-128 (1963)]. Despite these context effects, substantial improvements in category separability were observed when a pattern classifier incorporated spectral change information. Modeling work showed that many aspects of listener behavior could be accounted for by a fairly simple pattern classifier incorporating F0, duration, and two discrete samples of the formant pattern.     

Perception of front vowels: the role of harmonics in the first formant region

Vowel matching and identification experiments were carried out to investigate the perceptual contribution of harmonics in the first formant region of synthetic front vowels. In the first experiment, listeners selected the best phonetic match from an F1 continuum, for reference stimuli in which a band of two to five adjacent harmonics of equal intensity replaced the F1 peak; F1 values of best matches were near the frequency of the highest frequency harmonic in the band. Attenuation of the highest harmonic in the band resulted in lower F1 matches. Attenuation of the lowest harmonic had no significant effects, except in the case of a 2-harmonic band, where higher F1 matches were selected. A second experiment investigated the shifts in matched F1 resulting from an intensity increment to either one of a pair of harmonics in the F1 region. These shifts were relatively invariant over different harmonic frequencies and proportional to the fundamental frequency. A third experiment used a vowel identification task to determine phoneme boundaries on an F1 continuum. These boundaries were not substantially altered when the stimuli comprised only the two most prominent harmonics in the F1 region, or these plus either the higher or lower frequency subset of the remaining F1 harmonics. The results are consistent with an estimation procedure for the F1 peak which assigns greatest weight to the two most prominent harmonics in the first formant region.     

Relationship between fundamental and formant frequencies in voice preference

Covariation in the size of laryngeal and vocal tract structures leads to a moderate correlation between fundamental frequency (F0) and formant frequencies (FFs) in natural speech. A method of adjustment procedure was used to test whether listeners prefer combinations of F0 and FFs that reflect this covariation. Vowel sequences spoken by two men and two women were processed by the STRAIGHT vocoder to construct three sets of frequency-shifted continua. The distributions of "best choice" responses in all three experiments confirm that listeners prefer coordinated patterns of F0 and FF similar to those of natural speech.     

Testing theories of vowel inherent spectral change

Three competing accounts of vowel inherent spectral change in English all agree on the importance of initial formant frequencies; however, they disagree about the nature of the perceptually relevant aspects of formant change. The onset+offset hypothesis claims that the final formant values themselves matter. The onset+slope hypothesis claims that only the rate of change counts. The onset+direction hypothesis claims that only the general direction of change in formant frequencies is important. A synthetic-vowel perception experiment was designed to differentiate among the three. Results provide support for the superiority of the onset+offset hypothesis.     

Electron spin density distribution in the polymer phase of CsC60: assignment of the NMR spectrum

We present high resolution 133Cs-13C double resonance NMR data and 13C-13C NMR correlation spectra of 13C enriched samples of the polymeric phase of CsC60. These data lead to a partial assignment of the lines in the 13C NMR spectrum of CsC60 to the carbon positions on the C60 molecule. A plausible completion of the assignment can be made on the basis of an ab initio calculation. The data support the view that the conduction electron density is concentrated at the C60 "equator," away from the interfullerene bonds.     

Static, dynamic, and relational properties in vowel perception

The present work reviews theories and empirical findings, including results from two new experiments, that bear on the perception of English vowels, with an emphasis on the comparison of data analytic "machine recognition" approaches with results from speech perception experiments. Two major sources of variability (viz., speaker differences and consonantal context effects) are addressed from the classical perspective of overlap between vowel categories in F1 x F2 space. Various approaches to the reduction of this overlap are evaluated. Two types of speaker normalization are considered. "Intrinsic" methods based on relationships among the steady-state properties (F0, F1, F2, and F3) within individual vowel tokens are contrasted with "extrinsic" methods, involving the relationships among the formant frequencies of the entire vowel system of a single speaker. Evidence from a new experiment supports Ainsworth's (1975) conclusion [W. Ainsworth, Auditory Analysis and Perception of Speech (Academic, London, 1975)] that both types of information have a role to play in perception. The effects of consonantal context on formant overlap are also considered. A new experiment is presented that extends Lindblom and Studdert-Kennedy's finding [B. Lindblom and M. Studdert-Kennedy, J. Acoust. Soc. Am. 43, 840-843 (1967)] of perceptual effects of consonantal context on vowel perception to /dVd/ and /bVb/ contexts. Finally, the role of vowel-inherent dynamic properties, including duration and diphthongization, is briefly reviewed. All of the above factors are shown to have reliable influences on vowel perception, although the relative weight of such effects and the circumstances that alter these weights remain far from clear. It is suggested that the design of more complex perceptual experiments, together with the development of quantitative pattern recognition models of human vowel perception, will be necessary to resolve these issues.